This application focuses on the pathogenesis of smallpox, in response to RFA AI-02-002. Smallpox is a major bioterrorism threat because it is infectious by the aerosol route; once infection is established it spreads rapidly from human to human; it has a high case fatality rate among unvaccinated individuals; and there is no effective treatment. Since research on the pathogenesis of smallpox effectively ended more than 30 years ago, the knowledge base for designing new methods for prevention or treatment in light of the new threat of this disease is woefully out of date. All sequenced poxvirus genomes contain an open reading frame (orf) homologous to the ubiquitously expressed vertebrate plasma membrane protein CD47. Poxviruses express a number of proteins homologous to vertebrate proteins, including cytokine- and cytokine receptor-like genes, chemokine- and chemokine receptor-like genes, complement control proteins, and TNF receptor homologues. These viral proteins have been collectively called immuno-evasins because of their purpose to undermine normal host immune responses. In addition poxviruses express crmA and other proteins whose main purpose is to inhibit the death of infected cells. Both families of poxvirus proteins are important for viral survival, replication, and transmission. CD47 has several known functions that make it an ideal candidate for viral subversion, since it has a role in regulation of phagocytosis, phagocyte activation and migration, immune response to particulate antigens, and apoptosis of lymphocytes. The hypothesis of this application is that the highly conserved open reading frame in poxvirus genomes homologous to CD47 exists to subvert one or more of these CD47 functions to the advantage of the virus. Understanding the functions of this poxvirus CD47 homologue could lead to new approaches to prevention or therapy of smallpox. To understand the functions of this poxvirus orf, we propose to identify host cell ligands for the Variola and Vaccinia orfs homologous to CD47 and determine how the Variola CD47 homologue subverts normal CD47 functions. This increased understanding of this poxvirus homologue of mammalian CD47 will shed light on transmission and pathogenesis of smallpox and will aid in the development of new strategies for prevention and treatment of this disease.